Ans. In some complexes, the metal ion has no electrons in the d-orbitals (d0 configuration). Thus, when the visible light spectrum(white light) passes through the solution, there are no electrons to absorb the wavelength of the light. hence, all the seven rays of white light are transmitted giving white colour which means the complex is colourless.
If there are 10 electrons(d10 configuration) in the d-orbital of the complex, then all t2g and eg orbitals are fully occupied by the electrons. Therefore, d-d transitions of the electron is not possible and hence complex becomes colourless.